The present invention relates to measurement of systolic blood pressure and, in particular, it concerns a method for measuring systolic blood pressure using photoplethysmographic (PPG) sensors.
The assessment of the systolic and the diastolic arterial blood pressure has both physiological and clinical significance, and tremendous efforts have been applied to the development of a reliable noninvasive method for their measurement. Manual sphygmomanometry, which is based on an external cuff and audible detection of Korotkoff sounds, is considered to be the most accurate method, to which other methods should be compared, though it is prone to several sources of error. These include insufficient hearing acuity of the user, the auscultatory gap, and behavioral factors which influence the level of blood pressure, such as the presence of a physician. These errors are avoided when automatic measurement of the blood pressure is performed, but the available automatic noninvasive blood pressure (NIBP) meters and monitors have other sources of error, significantly reducing their accuracy.
Several methods have been suggested for automatic NIBP measurement. The most widely used of these, together accounting for about 96% of all NIBP monitors currently in use, are oscillometry and the auscultatory method. Oscillometry is based on the measurement of the change in the cardiac induced air pressure oscillations in the pressure cuff during cuff deflation after the elevation of the cuff air pressure above the systolic blood pressure. The cuff pressure at which the oscillometric pulse amplitude is maximal is generally regarded as the mean arterial pressure (MAP). The systolic (SBP) and the diastolic (DBP) blood pressures are determined from the envelope of the oscillometric curve using empirical criteria, such as the cuff pressure of maximal (or minimal) slope or of a determined value of the amplitude relative to maximal amplitude. These empirical criteria are the main source of error in oscillometry since they depend on the character of the cuff and since they are not appropriate for all patients. The automatic auscultatory method is also prone to artifacts due to external noise and vibrations.
The accuracy of the available automatic NIBP meters is very low, as can be understood from the standards imposed by the Association for the Advancement of Medical Instrumentation (AAMI) and the British Hypertension Society (BHS). Both standards are based on comparing the automated NIBP meter to manual sphygmomanometry (which is taken as a xe2x80x9cgold standardxe2x80x9d) for at least 85 subjects. The standards require that the mean difference between the SBP (or DBP) values measured by the sphygmomanometer and the device under examination do not exceed 5 mmHg, and that the standard deviation of that difference should not exceed 8 mmHg. In other words, a device for which 37% of the examinations differ from the reference manual sphygmomanometer by more than 8 mmHg and 5% of the examinations differ from the reference device by more than 16 mmHg is acceptable. Such low accuracy is permitted because the known methods are not capable of providing measurements of higher accuracy.
Both the auscultation method and oscillometry are based upon indirect physiological effects such as Korotkoff sounds, whose origin is not clear, or upon empirical parameters of the oscillometric envelope curve.
The volume-oscillometric method, like the oscillometric method, is based on the measurement of the arterial blood volume oscillation changes as a function of the pressure of an external cuff placed over the artery. The difference between the two methods is that in oscillometry the arterial volume oscillations are measured by means of pressure oscillations in the cuff itself, while in volume-oscillometry these oscillations are measured by photoplethysmography (PPG) or by another plethysmographic device placed under the cuff. As a result, in contrast to oscillometry, no oscillations are detected by the volume-oscillometry sensor, which is located at the distal end of the cuff, when the cuff pressure is above the systolic blood pressure. (In oscillometry the pressure in the cuff continues to oscillate even in high cuff pressure due to the impact of the arterial blood on the tissue under the proximal end of the cuff). Hence, volume oscillometry enables the measurement of systolic blood pressure more directly than oscillometry, with no need for empirical formula.
In principle, the measurement of systolic blood pressure using a pressure cuff and a PPG sensor is very straightforward. At cuff pressures below the SBP, some blood passes through the arteries under the cuff, producing pulsatile tissue blood volume variations which generate pulses in the PPG signal. Above the SBP, the artery under the cuff collapses, resulting in interruption of the pulsatile blood volume variations. FIG. 1 illustrates schematically a system for the measurement of the SBP using a pressure cuff 1 and a PPG sensor 2. A mercury manometer 3 is typically provided for calibration, while primary measurement and control of pressure in the cuff are performed, respectively, by a piezoelectric transducer 4 and a pressure pump (and its electronic control) 5. These cuff control components, as well as the electronic control 6 of the PPG device are all connected via an A/D card to a computer 7 which analyzes the results.
FIG. 2 shows typical curves of the PPG signal and of the air pressure as a function of time during the decrease in the cuff air pressure. Note the start of the PPG pulses when the air pressure decreases below SBP value.
Although theoretically simple, the actual measurement of SBP by this technique presents significant problems. Firstly, the amplitude of the first PPG pulses immediately after the air pressure falls below the SBP is small, often making it difficult to identify reliably from background noise in the PPG signal. More importantly, it has been found that, in 15-20% of subjects, no pulses are actually detected in the PPG signal until the pressure has decreased beyond the actual SBP (measured by Korotkoff sounds) by as much as 10-20 mmHg. Without in any way limiting the scope of the present invention, this latter problem is believed to be attributable to the mechanical pressure applied by the PPG sensor itself on the arteries underneath. When the cuff air pressure is above SBP, the arteries distal to the cuff drain into the veins so that the arterial blood pressure becomes relatively low and the small pressure exerted by the sensor can make them collapse (close them). In some cases the small blood volume pulses entering the arteries distal to the cuff when the cuff air pressure is just below SBP value cannot open the arteries under the sensor.
Finally, reference is made to U.S. Pat. No. 5,447,161 to Blazek et al. which proposes a method for measuring venous blood pressure, SBP and DBP during a slow increase (4 mmHg/s) in applied cuff pressure. When the applied external pressure is above the venous blood pressure, the veins collapse. As a result, the slow air pressure increase leads to accumulation of much blood in the hand vascular system. As will be detailed below, the slow increase leads to reduction in the distal PPG signal amplitude so that the PPG signal may initially be undetectable when the cuff air pressure decreases to slightly below the SBP. This renders SBP measurement unreliable.
There is therefore a need for a method for measuring arterial systolic blood pressure using a cuff and a PPG sensor which would provide accurate and reliable results.
The present invention is a method for measuring arterial systolic blood pressure in a subject.
Conceptually, the present invention provides two approaches to overcoming the problem of delayed PPG sensing of cardiac induced pulsatile variations in tissue blood volume just below the systolic blood pressure.
According to a first, particularly simple approach, the form of attachment of the PPG sensor is modified to minimize pressure exerted on the adjacent tissue. Thus, according to one preferred implementation, a PPG sensor is attached to the fingernail of a subject, typically by use of double-sided, transparent adhesive tape. The contact surface of the PPG sensor is modified to provide a curvature approximating to that of a typical fingernail. In other respects, the PPG sensor is a conventional sensor of either the transmission type or of the reflection type.
Although this first approach has been found to provide very accurate and reliable SBP measurements under controlled conditions, it is thought that the lack of significant contact pressure would lead to problems of signal instability in cases where the subject does not remain still. This is particularly true for applications with significant motion such as during stress tests or when the patient is being transferred by ambulance.
To avoid these practical limitations, the present invention also provides a second approach which is believed to be suitable for a wider range of applications and to provide highly accurate and reliable SBP measurements. According to this second approach, the cuff pressure increase before it reaches SBP value is performed in such a manner as to cause venous occlusion for a period of time, long enough to sufficiently increasing venous blood volume and pressure. The latter prevents the drainage of the arteries of their blood, so that the arteries remain open even under the mechanical pressure of the PPG sensor. This has been found to avoid the problem of failure to detect the PPG pulses at pressures just below the SBP.
According to the aforementioned approach, the cuff air pressure increase should last for long enough period of time in order to prevent emptying of the arteries after the closure of the arteries under the cuff. If the cuff air pressure increases slowly up to the value of DBP or lower, and then increases fast to above SBP value, no significant change in the blood pressure in the distal arteries is expected. If, however, the cuff air pressure slowly increases above the DBP value, the mean arterial blood pressure will increase. By way of illustration, when the cuff air pressure is above DBP, say 100 mmHg (for a subject whose SBP/DBP values are 120/80 mmHg), the artery under the cuff opens during the part of the systole for which the arterial blood pressure is above 100 mmHg, and blood enters the arteries distal to the cuff. During diastole, if the arterial blood pressure distal to the cuff remains above 100 mmHg, blood drains back from these arteries through the cuff. When the arterial blood pressure is below 100 mmHg, the artery under the cuff collapses, so that the blood pressure in the arteries distal to the cuff is remains at least 100 mmHg. This occurs if the increase of the cuff air pressure is slow enough.
Hence, the manner of increasing pressure in the cuff should be so chosen to prevent the mean blood pressure approaching the systolic blood pressure, in order to avoid the problem of reduced amplitude of PPG pulses due to distended blood vessels. For higher blood pressure, the arteries walls are more distended and their compliance (the increase of the arterial blood volume for a given increase of blood pressure) is low. By way of illustration, FIG. 3 shows the relationship between the volume of the arteries and their transmural blood pressure, i.e. the difference between the arterial blood pressure and the external pressure. It can be seen that blood volume increase due to a given blood pressure increase is smaller for higher blood pressure. As a result, when the cuff air pressure decreases to slightly below the SBP (after slow increase of the cuff air pressure which causes the mean arterial blood pressure to be relatively high), the small changes in arterial blood pressure will result in very small changes in the arterial blood volume such that the pulses in the PPG signal may be masked by the existing background noise.
The aforementioned U.S. Pat. No. 5,447,161 to Blazek et al. suffers from exactly this problem. Because of the slow increase (4 mmHg/s) in the applied cuff pressure, much blood accumulates in the hand and arm vascular system, so that the mean blood pressure approaches the systolic blood pressure and blood flow into the arteries distal to the cuff is greatly reduced. This presents problems for the SBP measurement which is performed by detecting the start of blood flow when the cuff air pressure decreases below SBP value. By way of illustration, we performed tests in which we measured the systolic blood pressure by the auscultatory method for a set of 19 subjects using fast increase and slow increase of air pressure. The measurement was performed during the subsequent slow decrease of the cuff air pressure. Out of the 19 subjects, the SBP value obtained after the slow increase for 18 of them was lower than that obtained after the fast air pressure increase. This illustrates clearly that, after slow increase of the cuff air pressure of the type proposed by Blazek et al., the blood flow through the artery under the cuff can be detected only when the cuff air pressure has already been lowered significantly below the SBP value.
Because of the more general applicability of this second approach, it is this approach which will form the basis of the various implementations which will be detailed in the following description.
Hence, according to the teachings of the present invention there is provided, a method for measuring systolic blood pressure in a subject, the method comprising: (a) generating a first signal indicative of cardiac induced pulsatile variations in tissue blood volume in a first region of the subject""s body; (b) applying a pressure cuff to a second region of the subject""s body proximal with respect to the first region; (c) increasing a pressure within the pressure cuff to a pressure sufficient to prevent sensing of cardiac induced pulsatile variations in tissue blood volume in the first region; (d) reducing a pressure within the pressure cuff; and (e) identifying as a systolic blood pressure of the subject a cuff pressure at which the cardiac induced pulsatile variations in tissue blood volume in the first region are found to restart, wherein the increasing of pressure within the pressure cuff is performed such that it takes at least 10 seconds for the cuff pressure to reach 100 mmHg, and such that the average rate of increase is at least 20 mmHg per second from 100 mmHg up to the pressure sufficient to prevent sensing of cardiac induced pulsatile variations.
According to a further feature of the present invention, the identifying includes: (a) integrating the first signal with respect to time over the length of a suspected pulse to derive an area under the pulse; and (b) identifying the suspected pulse as a restarting of cardiac induced pulsatile variations in tissue blood volume only if the area exceeds a given minimum value.
According to a further feature of the present invention, the identifying includes: (a) deriving a maximum gradient of the first signal with respect to time over the length of a suspected pulse; and (b) identifying the suspected pulse as a restarting of cardiac induced pulsatile variations in tissue blood volume only if the maximum gradient exceeds a given minimum value.
According to a further feature of the present invention, the method further includes: (a) generating a second signal indicative of cardiac induced pulsatile variations in a third region of the subject""s body; (b) calculating a delay between features of the first and the second signals indicative of corresponding cardiac induced pulsatile variations; and (c) evaluating a baseline value of the delay when the pressure within the pressure cuff is substantially zero, and calculating a corrected delay by deduction of the baseline value from measurements of the delay, wherein the identifying includes identifying a suspected pulse as a restarting of cardiac induced pulsatile variations in tissue blood volume only if the value of the delay is between 100-250 ms.
According to a further feature of the present invention, the method also includes: (a) generating a second signal indicative of cardiac induced pulsatile variations; (b) deriving empirically an approximate relationship between values of a delay of the first signal after the second signal and the cuff pressure; and (c) identifying a suspected pulse as a restarting of cardiac induced pulsatile variations in tissue blood volume only if the suspected pulse occurs within a given time window after a pulse of the second signal, the given time window being derived from the approximate relationship evaluated at an expected value of systolic blood pressure.
There is also provided according to the teachings of the present invention, a method for measuring systolic blood pressure in a subject, the method comprising: (a) generating a first signal indicative of cardiac induced pulsatile variations in tissue blood volume in a first region of the subject""s body; (b) applying a pressure cuff to a second region of the subject""s body proximal with respect to the first region; (c) increasing a pressure within the pressure cuff to a pressure sufficient to prevent sensing of cardiac induced pulsatile variations in tissue blood volume in the first region; (d) reducing a pressure within the pressure cuff, and (e) identifying as a systolic blood pressure of the subject a cuff pressure at which the cardiac induced pulsatile variations in tissue blood volume in the first region are found to restart, wherein the increasing of pressure within the pressure cuff is performed at a substantially constant rate chosen such that it takes at least between 10 and 30 seconds for the cuff pressure to reach 160 mmHg.
There is also provided according to the teachings of the present invention, a method for measuring systolic blood pressure in a subject, the method comprising: (a) generating first and second signals indicative, respectively, of cardiac induced pulsatile variations in tissue blood volume in a first region and a second region of the subject""s body; (b) processing the first and second signals to derive values of a delay between pulses in the first signal and corresponding pulses in the second signal; (c) applying a variable pressure to a third region of the subject""s body proximal with respect to the first region so as to affect blood flow through at least one artery in the third region, the variable pressure being varied as a function of time, the first, the second and the third regions being chosen such that the delay varies as a function of the variable pressure; (d) raising the variable pressure at a rate which changes as a function of the delay until the first signal no longer provides identifiable pulses corresponding to cardiac induced pulsatile variations in tissue blood volume; and (e) identifying as the arterial systolic blood pressure the value of the variable pressure corresponding to a boundary below which the first signal includes identifiable pulses corresponding to cardiac induced pulsatile variations in tissue blood volume.
According to a further feature of the present invention, a baseline value of the delay is evaluated when the variable pressure is substantially zero, and a corrected delay is calculated by deduction of the baseline value from measurements of the delay.
According to a further feature of the present invention, the rate is approximately 2 mmHg per second at least for values of the corrected delay below about 50 ms.
According to a further feature of the present invention, the rate is at least about 5 mmHg per second at least for values of the corrected delay between about 50 and about 100 ms.
According to a further feature of the present invention, the rate is approximately 2 mmHg per second at least for values of the corrected delay in excess of about 100 ms.
According to a further feature of the present invention, the boundary is identified during the raising of the variable pressure.
According to an alternative feature of the present invention, the boundary is identified during a subsequent step of reducing the variable pressure.